ROCKVILLE, Md. (DTN) -- Not so long ago, insecticide and fungicide use in soybeans was not very common.

Even after soybean aphids invaded the Midwest, fewer than 10% of soybean acres received an insecticide or fungicide application most years before 2008. Now, neonicotinoid insecticides and fungicides coat soybean seeds planted to tens of millions of acres across the Midwest and South -- more than 50% of all soybean acres, by some industry estimates.

Are they always necessary and worthwhile? Not really, says a new meta-study published by nearly two dozen scientists across the Midwest, Mid-Atlantic and Southeast U.S.

"It's a huge dataset from a wide swath of the major northern and eastern soybean growing regions of the U.S.," said University of Wisconsin soybean and small grains specialist Shawn Conley, who helped lead the research. The study used data on seed treatments from 194 academic studies, which covered 12 years and 14 states, accounting for about 85% of U.S. soybean acreage. With the exception of Arkansas, most of the Midsouth states, where pest pressure is generally more intense, were not included.

"Overall, it shows that while we see a small general yield benefit, usually around 2 bushels per acre (bpa), when it comes down to economics, there's really no farm-gate value there," Conley said.

At issue is the fact that the pests targeted by a key ingredient in seed treatments, neonicotinoid insecticides, are not consistently present in many Midwest fields, explained Purdue University entomologist Christian Krupke, who contributed to the study.

A long list of pests are labeled as targets of these seed treatments, from white grubs and wireworms to bean leaf beetles and soybean aphids. But research led by USDA entomologists on historic pest pressures in the Soybean Belt suggest most of these insects do not occur regularly enough in the majority of fields to warrant widespread use of an annual insecticide, Krupke noted. (See the research here: https://academic.oup.com/…).

The exception is soybean aphid, which can be an annual pest. However, past university research suggests neonicotinoids don't persist long enough in soybean tissue to target aphid build-ups when they normally occur, except in late-planted soybean fields.

"So when you have very few pests and really short-lived pesticides, that equation makes it very hard to get an economic benefit," Krupke said.

In contrast, the diseases targeted by fungicide seed treatments are more reliably present in most fields, noted University of Kentucky plant pathologist Carl Bradley, who also contributed data to the study. But the primary result of disease in soybean seedlings -- reduced stands -- doesn't always translate to reduced yields, he said.

"Soybean plants are really good at compensating for missing plants," he explained. "If there is a plant missing from disease, its neighbor will grow to fill that area. That's probably why we don't always see a yield response with fungicide seed treatments."

Some factors make a yield response more likely, such as early planting into cool, wet soils that favor damaging diseases like Sudden Death Syndrome, he noted. But because of the current structure of seed production, farmers have to choose whether to treat their seed before they know how the planting season will go.

"The problem with seed treatments is that you have to make a decision before you really know a lot of factors, like planting date," Bradley said.

Overall, the scientists found that the use of a fungicide seed treatment alone did not produce any significant yield response compared to untreated seed in their meta-study. When a fungicide and neonicotinoid seed treatment were combined, researchers saw overall yield benefits ranging between 0.2 and 3 bpa -- not enough consistency to warrant any additional cost in seed prices, Conley said.

To make that conclusion on cost, the study's scientists had to do some statistical gymnastics, since companies are usually not willing to share the exact cost they charge per seed treatment.

"So what we did was look at it from the other side: How much would it be worth for you to pay for treated seed, based on the yield benefits," explained Conley. "And what it came down to was that you should pay no more than zero dollars more for treated seed."

The study found that only if soybean prices were to exceed $20 per bushel -- more than double their current price -- would it be worthwhile for a grower to pay extra for a seed treatment.

Other soybean management practices, such as planting date, row spacing and seeding rate have "greater potential to increase soybean yields across the entire examined region" than seed treatments, the study's authors noted.

Add to this a growing body of research showing possible effects of neonicotinoids on non-target organisms, such as honeybees, beneficial insect predators and soil-decomposing insects, and the continued widespread use of seed treatments is not beneficial for farmers, the scientists concluded.

"The lack of consistent economic yield benefits attributable to NST [neonicotinoid seed treatments], coupled with mounting reports of potential environmental risks, highlight that the current default approach of prophylactic applications of NST in soybeans in the U.S. should be re-evaluated," they wrote.

Seed treatments remain an important tool, but they should be deployed on a field-by-field basis, Conley added. They can be very valuable for growers with a known history of pests, like bean leaf beetle, or those planting into known disease- and insect-friendly conditions. And some university entomologists in the Midsouth maintain seed treatments are economically beneficial to growers there, where pest pressure can be more intense and prolonged year-round.

However, the use of them as an annual insurance policy across tens of millions of acres is not sustainable, Krupke said.

"We'll need to use pesticides in agriculture for the foreseeable future," he said. "But to use a pesticide when we don't really need it is problematic from a resistance, profit and environmental standpoint. And that's what we keep trying to use data to highlight."